Various antenna devices are now available for vehicle mounted use. As such an antenna device, there is known, for example, an AM/FM radio antenna capable of receiving AM broadcast and FM broadcast. In general, as the AM/FM radio antenna, a rod antenna is used. The rod antenna is constituted by an element portion having an element (helical element) including a helical conductor covered by a cover and a base portion for attachment of the element portion.
In a state where the rod antenna is attached to a vehicle body, the element portion significantly protrudes from the vehicle body, which may impair an outer appearance and design of the vehicle, which may be broken at the time of parking or car washing, and which may be in danger of theft because the rod antenna is mounted outside the vehicle.
Under such circumstances, there is proposed a low-profile antenna device having a configuration in which the entire height of the antenna device is made lower than that of the rod antenna, the element is housed in an antenna case to prevent an element from being exposed outside, and the antenna case is formed into a shark-fin shape in consideration of design of the entire vehicle after attachment of the antenna. Many low-profile antenna devices having such a configuration have a height of 70 mm or lower and a longitudinal length of around 200 mm in terms of regulatory requirements.
However, the low-profile antenna device having a height as low as 70 mm or less may degrade radiation efficiency due to antenna conductor loss (reduction in element length), which may cause sensitivity degradation. For example, Patent Document 1 discloses an antenna device aiming to solve this problem. In the antenna device disclosed in Patent Document 1, an antenna substrate having an antenna pattern formed thereon and having a coil for antenna inductance correction between the antenna pattern and a feeding point is vertically arranged on a base portion, and a hat-shaped top portion is disposed at an upper end of the antenna substrate so as to straddle the antenna substrate.